Join table into different columns

Question

I have two tables (simplified).

table1:  
 - id (serial)

table2:  
 - table1_id (integer)  
 - value (text)  
 - type (integer)

I want to join these two tables, but depending on the type, I want to join the "value" of table2 into different columns. Result should look like this:

id | table2value0 | table2value1 | table2othervalues

table2value0 is the value from table2 where type is 0.
table2value1 is the value from table2 where type is 1.
table2othervalues are the remaining values where type is something else, as an array.

If there are more than one value for table2value0 and table2value1, I only want the first. Order doesn't matter, so the order it was inserted.

If some entries in table1 do not have matches in table2, they should appear in the output as NULL.

score 4 · Answer 1 · edited May 23 '17 at 11:33

Assuming Postgres 9.4 or later:

SELECT id, t2.table2value0, t2.table2value1, t2.table2othervalues
FROM   table1 t1
LEFT   JOIN (
   SELECT table1_id AS id
        , min(value) FILTER (WHERE type = 0) AS table2value0
        , min(value) FILTER (WHERE type = 1) AS table2value1
        , array_agg(value) FILTER (WHERE type IN (1,2) IS NOT TRUE) AS table2othervalues
   FROM   table2
   GROUP  BY table1_id
   ) t2 USING (id);

The aggregate FILTER clause was introduced with pg 9.4. There are various less elegant alternatives for older versions:

Postgres Nested WHEN Aggregate Function

This returns one row for every row in table1, missing types in table2 result in NULL values.

If there are more than one value for table2value0 and table2value1, I only want the first.

You seem to be under the impression that there is a natural order of rows, but there is not:

Does Postgres preserve insertion order of records?

I picked the minimum value instead, since your requirement is effectively undefined.

The best query depends on your actual table definition (including all constraints).

In particular, the expression type IN (1,2) IS NOT TRUE also catches type IS NULL, which may not be necessary, depending on your missing table definition.

PostgreSQL Crosstab Query

Join table into different columns

1 Answers1